This invention relates to velocity sensing in general and more particularly to a radar velocity sensing technique used in aircraft navigation and the like of an improved type which permits the sensing of not only velocity but the sense of direction of velocity.
In my prior co-pending U.S. application, Ser. No. 334,011, filed Feb. 20, 1973, now Pat. No. 3,838,424, issued Sept. 24, 1974, I disclosed a velocity sensing apparatus in which a single radiating horn illuminates a spot on the ground from which power is backscattered to a pair of spaced receiving antennas. Because of the relative motion between the transmitting antenna and the ground, the backscattered radar speckled pattern which is developed moves as a whole in the opposite direction permitting the disclosed system to make use of that fact and measure the time required for the pattern to traverse the distance between the two receiving antennas by determining the frequency at which there is equal power in the sum and difference of the outputs of a quadratic detector coupled to each channel. Also disclosed is an additional antenna permitting measuring velocity in two directions by tracking the output of each of two pairs of the three antennas and providing them as inputs to a resolver system.
Although the disclosed system operates quite well, it is intended primarily for use in aircraft or other vehicles which generally travel in only one direction, e.g. such as a fixed wing aircraft always travels forward and never backward. In a vehicle like a fixed wing aircraft, the velocity along heading and across heading are the quantities measured. Alternatively, a system of this type can measure velocity along heading and drift angle. When used in such an application, there is no need for the system to be able to determine the sense of direction, i.e., whether it is forward or backward, since it is a preestablished fact that motion is always forward. However, if such a system is applied to another type vehicle, such as a helicopter which can travel backward, forward and sideways, the ability to detect the sense of direction becomes important.
In view of this, the need for an improved system of the type previously disclosed is evident.